Internal combustion engines tend to generate a variety of exhaust emissions during operation. Aftertreatment systems are used with most modern internal combustion engines to eliminate or reduce certain of these emissions. Over the years, many different aftertreatment strategies have been proposed for reducing or eliminating emissions such as particulates, unburned hydrocarbons and nitrogen oxides of various types, collectively referred to as “NOx.” Particulate filters, catalytic converters and other devices are familiar examples of aftertreatment components directed to emissions reduction. Aftertreatment components include devices which are adapted to store or “trap” emissions, and devices which convert certain emissions into other materials considered less harmful or more manageable, as well as systems which do both.
A device commonly known as a diesel particulate filter is often used in connection with compression ignition diesel engines to trap particulates at a location downstream from the engine, rather than releasing the particulates via the tailpipe or exhaust stack. While diesel particulate filters have been demonstrated to be effective in reducing the release of undesired particulates in exhaust, over time the filter tends to become clogged. As the filter becomes progressively more clogged, it can create undesired backpressure in the exhaust system and become less effective. Most engines equipped with a particulate filter are also equipped with some means for burning off or “regenerating” the particulates trapped in the filter. One conventional strategy for filter regeneration is to induce combustion of the accumulated particulates, cleaning the filter. “Active” regeneration techniques for initiating combustion of accumulated particulates include injection of additional fuel into the exhaust system itself, heating the filter via electric heaters and the like and other strategies such as operating the engine via post injections to raise exhaust temperatures to temperatures sufficient to initiate combustion of the accumulated particulates.
While the previously mentioned regeneration strategies have seen success, they tend to require additional hardware and/or energy to operate. An alternative regeneration strategy is known in the art as continuous regeneration, and typically utilizes a catalyst positioned within the exhaust system at a location upstream from a particulate filter. Such catalysts may be used to chemically convert certain exhaust constituents into different chemical compounds which are capable of passively inducing regeneration of the accumulated particulates. In one common example, a diesel oxidation catalyst which includes platinum is positioned within the exhaust stream at a location upstream from a diesel particulate filter. As exhaust gases pass through the diesel oxidation catalyst, nitrogen oxide or “NO” in the exhaust gases may be converted to nitrogen dioxide or “NO2”, which in turn oxidizes particulate matter trapped in the particulate filter.
Engineers have developed various strategies for varying the catalyst loading density and uniformity within passively regenerated systems to result in optimal particulate matter oxidation. Other known strategies place catalysts on the diesel particulate filter itself, such that NO in the exhaust gases is continuously converted to NO2 as the exhaust gases pass through the filter, ostensibly providing a continuous supply of NO2 for continuous oxidation of accumulated particulate matter. Such systems have proven applicable in certain environments, but they are not without problems. While diesel oxidation catalysts may be effectively used to oxidize accumulated particulates for filter regeneration, NO2 may be generated in excess, potentially creating problems with other components of the aftertreatment system positioned downstream.
U.S. Pat. No. 6,805,849 to Andreasson et al. is directed to one type of exhaust aftertreatment system adapted to adjust an NO2 to NO ratio to purportedly improve NOx reduction. In particular, Andreasson et al. propose an improved SCR catalyst system adapted to supply exhaust gas at a particular NO2 to NO ratio to improve NOx reduction. A shortcoming of the design proposed by Andreasson et al. is that a relatively bulky and expensive diesel oxidation catalyst and associated apparatus appears to be required.